Playing on the nerves of an audience who just watched Cary Elwes commit self-harm, the first Saw's finale rapidly recaps the previous 80 minutes in flashbacks, with snippets of dialogue that reveal that Zeb, who we thought was the killer, is just another pawn. The mastermind -- surprise! -- is actually John, one of Dr. Gordon's cancer patients, who has applied prosthetics to his head so that he can pretend to be a dead body in the room.
The movie ends before giving the audience time to reflect on the two protagonists' idiocy. A very conscious guy with fake brains glued to his bald head? OK, I'd believe a healthy man could pretend to be dead while in a room with people for eight hours, but not someone as terminally ill as Jigsaw.